In recent years, with the rapid spread of small electronic equipment such as a mobile phone and a notebook-sized personal computer, demand for a non-aqueous electrolyte secondary battery which is used as a chargeable and dischargeable power supply has been rapidly increased. As a positive electrode active material for a non-aqueous electrolyte secondary battery, lithium-nickel composite oxide represented by lithium-nickel dioxide (LiNiO2) and lithium-manganese composite oxide represented by lithium-manganese dioxide (LiMnO2) have been wildly used as well as lithium-cobalt composite oxide represented by lithium-cobalt dioxide (LiCoO2).
However, there are some defects in the lithium cobalt dioxide, such that the lithium cobalt dioxide is expensive because its reserve is a little in the earth, and that the lithium cobalt dioxide contains cobalt which is unstable in supply and has a highly fluctuating price range, as a major component. Therefore, there have been remarked lithium-nickel composite oxide containing relatively inexpensive nickel as a major component and lithium-manganese composite oxide containing relatively inexpensive manganese as a major component from the viewpoint of cost. The lithium manganese dioxide is superior in thermal stability to lithium manganese dioxide. However, the lithium manganese dioxide has some problems when the lithium manganese dioxide is practically used in a battery, because its charge and discharge capacity is much smaller than that of the other materials, and its charge and discharge cycle characteristic showing the life of a battery is much shorter than that of the other materials. On the other hand, since the lithium nickel dioxide has a charge and discharge capacity greater than the lithium cobalt dioxide, the lithium nickel dioxide has been expected to be used as a positive electrode active material which enables to produce an inexpensive battery having a high energy density.
The lithium nickel oxide is usually produced by mixing a lithium compound with a nickel compound such as nickel hydroxide or nickel oxyhydroxide, and calcining them. The form of the lithium nickel oxide is powder in which primary particles are mono-dispersed or powder of secondary particles of aggregated primary particles having spaces between the primary particles. However, both powders have some defects such that the powders are inferior in thermal stability under the state of charge to the cobalt lithium dioxide. Specifically, since pure lithium nickel dioxide has a defect in thermal stability, and charge and discharge cycle characteristic, the lithium nickel dioxide cannot be used in a practical battery. This is because the stability of the lithium nickel dioxide in crystal structure under the state of charge is inferior to that of the lithium cobalt dioxide.
In order to eliminate this defect, it has been commonly carried out that a part of nickel is replaced with a transition metal element such as cobalt, manganese or iron, or an element which is different from nickel, such as aluminum, vanadium or tin, to stabilize the crystal structure in the state such that lithium is fell out by charging, to give lithium-nickel composite oxide having suitable thermal stability and charge and discharge cycle characteristic, which can be used as a positive electrode active material (see, for example, Non-patent Literature 1 and Patent Literature 1).
However, the replacement of the nickel with an element in a small amount causes insufficient improvement in thermal stability, and the replacement of the nickel with an element in a large amount causes lowering in electric capacity. Therefore, the advantageous merits of the lithium-nickel composite oxide cannot be sufficiently imparted to a battery.
Also, in order to reduce the reactivity of a positive electrode active material and an electrolyte in a battery, there has been proposed a method which includes reducing the specific surface area of a positive electrode active material which is used for the purpose of reducing its reaction area, to improve thermal stability (see, for example, Patent Literature 2). This method has been found on the basis such that thermal stability is improved by introducing aluminum and yttrium in a small amount into lithium composite oxide, and that the reactivity of an anode material and an electrolyte under overcharge can be suppressed by reducing the specific surface area of the lithium composite oxide.
However, since the above-mentioned method is focused on the specific surface area in terms of the reaction area between the positive electrode active material and the electrolyte under the condition where impurities or by-products are attached to the surface of the particles of the positive electrode active material, the specific surface area used in the above-mentioned method does not reveal an exact reaction area.
As a method for reforming a positive electrode active material, Patent Literature 3 proposes a method for removing impurities or by-products which are attached to the positive electrode active material by washing with water under a specific condition. Patent Literature 3 refers to the effect for improving thermal stability by the change of the specific surface area of the powder after washing with water, and DSC calorific value.